Position sensors such as encoders and Hall-effect sensors provide motor position information. Motor operation depends on accurate motor position information, especially for permanent magnet brushless motors. For instance, phase excitation for torque production is controlled according to motor position information.
Commutation (i.e., phase excitation) of the stator windings of a motor is controlled according to detected position of the rotor of the motor. Calibration of rotor position relative to the stator is done at a production facility through end of line calibration. Such calibration includes calibration of rotor position offset relative to the stator magnetic field. The motor leads (e.g., three phase) are designated or color coded for proper connectivity to electronic controller terminals. Wrong connections lead to improper calibration and operation of the motor. A problem is that ordinary calibration processes require relatively much effort and expense.
Some motor systems include a magnetic rotary encoder to monitor the rotary position of the rotor relative to the stator. A magnetic rotary encoder typically includes a relatively small two pole encoder magnet. The encoder magnet is mounted onto the rotor shaft to rotate as the rotor rotates. A magnetic sensor of the encoder detects the rotary position of the encoder magnet. The detected rotary position of the encoder magnet corresponds to the rotary position of the rotor when the encoder magnet is aligned with a reference point of the rotor from which the rotary position of the rotor is to be measured.
However, the alignment between the encoder magnet and the rotor is arbitrary upon the encoder magnet being mounted onto the rotor shaft during motor manufacture. As such, the encoder magnet is aligned with an arbitrary point of the rotor as opposed to the reference point of the rotor. Calibration is performed to identify the offset in alignment of the encoder magnet, which is aligned with the arbitrary point of the rotor, relative to the reference point of the rotor. The summation of the detected rotary position of the encoder magnet and the offset in alignment corresponds to the rotary position of the rotor.
For instance, the arbitrary point of the rotor is 40° apart from the reference point of the rotor in the clockwise direction. Hence, the arbitrary point leads the reference point by 40°. Thus, the detected rotary position of the encoder magnet (for example, 150°) is +40° more than the rotary position of the rotor (in this example, 110°). Therefore, +40° is subtracted from the detected rotary position of the encoder magnet to obtain the rotary position of the rotor.
The relationship of the summation of the detected rotary position of the encoder magnet and the offset in alignment corresponding to the rotary position of the rotor holds as long as the encoder magnet is aligned with the arbitrary point. A problem is that the orientation of the encoder magnet may change with respect to the rotor shaft during motor operation. For instance, the encoder magnet may become loosened and move during the lifetime of the motor. As a result, the encoder magnet moves out of alignment with the initial arbitrary point of the rotor and becomes aligned with some other arbitrary point of the rotor (which could even be the reference point of the rotor). As a result, the offset in alignment of the encoder magnet relative to the reference point becomes different than the offset originally calibrated.
Proper motor operation cannot occur without accurate rotary position of the rotor as motor commutation is controlled according to the rotary position of the rotor. Further, sometimes it may not be discernible that the encoder magnet has moved out of the calibrated alignment with the rotor. As such, motor operation based on the resulting faulty rotary position information provided by the encoder may be understood to be proper when in fact the motor operation is improper.